A conventional traction control apparatus has been known to eliminate a slip of a driving wheel of a vehicle when the vehicle starts or accelerates particularly while the vehicle is running on a low-friction road surface such as a frozen, snow-covered or muddy one, so that a driving stability and a starting/accelerating performance can be improved. In such an apparatus as disclosed in Unexamined Japanese Patent Application (OPI) No. 61-85248, each of slip conditions of left and right driving wheels can be detected, and when the extent of the slip exceeds a predetermined value, a braking operation is controlled by increasing a brake fluid pressure applied to the brake device for the driving wheel and, simultaneously, a throttle operation is controlled to thereby decrease an output of an engine.
According to the conventional throttle operation control, a difference in speed between the driving wheel and the non-driving wheel is determined as one factor, and an acceleration degree of the driving wheel is determined as another factor. The throttle operation is controlled by using a throttle control map from which an open-and-close speed of the throttle according to the difference in the wheel speeds and the acceleration degree can be determined.
However, in such a conventional throttle operation control using the control map thus operated, the throttle operation can hardly be controlled with high accuracy corresponding to actual conditions of the vehicle and the road surface.
In another conventional throttle operation control as disclosed in U.S. Pat. No. 4,736,814, it is proposed that an open-and-close speed of a throttle valve is controlled in accordance with a comparison of a corrected driving wheel speed and a reference speed. However, the open-and-close speed of the throttle valve is not controlled in view of a friction coefficient of a road surface so that optimum control cannot be attained.